Mass spectrometry is a method of analyzing gas-phase ions generated from a particle molecular sample. The gas-phase ions are separated in electric and/or magnetic fields according to their mass-to-charge ratio. Analyzing molecular weights of samples using mass spectrometry consists mainly of three processes: generating gas phase ions, separating and analyzing the ions according to their mass-to-charge ratio and detecting the ions. A mass spectrometer is an instrument for implementing these processes to measure the gas-phase mass ions or molecular ions in a vacuum chamber via ionizing the gas molecules and to measure the mass-to-charge ratio of the ions.
Formation of gas phase samples ions is an essential process for the operation of a mass spectrometer. There are many ionization methods and related sources suitable for different kinds of samples. For example, ions may be generated by electron ionization (EI) in vacuum. EI is the most appropriate technique for relatively small (m/z<700) neutral organic molecules that can easily be promoted to the gas phase by heating without decomposition (i.e. volatilization). Electron ionization is achieved through the interaction of an analyte with an energetic electron beam resulting in the loss of an electron from the analyte and the production of a radical cation. Electrons are produced by thermionic emission from a tungsten or rhenium filament. These electrons leave the filament surface and are accelerated towards the ion source chamber, which is held at a positive potential (equal to the accelerating voltage). The electrons acquire energy equal to the voltage, which typically is about 70 electron volts (70 eV), between the filament and the source chamber.
Chemical ionization (CI) is another process for formation of ions. In contrast to EI, most applications of CI produce ions by the relatively gentle process of proton transfer. The sample molecules are exposed to a large excess of ionized reagent gas. Transfer of a proton to a sample molecule M, from an ionized reagent gas such as methane in the form of CH5+, yields the [M+H]+positive ion. Negative ions can also be produced under chemical ionization conditions. Transfer of a proton from M to other types of reagent gas or ions can leave [M−H]−, a negatively charged sample ion.
Another ion formation process is based on corona discharge ionization. Corona discharge ionization is an electrical discharge characterized by a corona. Corona discharge ionization occurs when one of two electrodes placed in a gas (i.e. a discharge electrode) has a shape causing the electric field on its surface to be significantly greater than that between the electrodes. Corona discharges are usually created in gas held at or near atmospheric pressure. Corona discharge may be positive or negative according to the polarity of the voltage applied to the higher curvature electrode i.e. the discharge electrode. If the discharge electrode is positive with respect to the flat electrode, the discharge is a positive corona, if negative the discharge is a negative corona.
Desorption ionization is a term used to describe the process by which a molecule is both evaporated from a surface and ionized. In field desorption (FD), the sample is coated as a thin film onto a special filament placed within a very high intensity electric field. In this environment, ions created by field-induced removal of an electron from the molecule are extracted into the mass spectrometer. Samples are desorbed and ionized by an impact process that involves bombardment of the sample with high velocity atoms, ions, fission fragments, or photons of relatively high energy. The impact deposits energy into the sample, either directly or via the matrix, and leads to both sample molecule transfer into the gas phase and ionization. Fast atom bombardment (FAB) involves impact of high velocity atoms on a sample dissolved in a liquid matrix. Secondary ion mass spectrometry (SIMS) involves impact of high velocity ions on a thin film of sample on a metal substrate or dissolved in a liquid matrix. Plasma desorption (PD) involves impact of nuclear fission fragments, e.g. from 252Cf, on a solid sample deposited on a metal foil. Matrix assisted laser desorption ionization (MALDI) involves impact of high energy photons on a sample embedded in a solid organic matrix. Most desorption ionizations undergo in vacuum system, in which molecules embedded on a substrate and introduced are desorbed and ionized using energetic charged particles or laser beams.
Other processes for ion formation are also known. For example, atmospheric pressure ionization (API) can generate sample ions from liquid solution in atmospheric pressure. Electrospray ionization (ESI), introduced by Fenn et al., is a widely used method to produce gaseous ionized molecules desolvated or desorbed from a liquid solution by creating a fine spray of droplets in the presence of a strong electric field. The ESI source consists of a very fine metal emitter or needle, a counter electrode and a series of skimmers. A sample solution is sprayed into the source chamber to form droplets. The droplets carry charge when the exit the capillary and as the solvent vaporizes the droplets disappear leaving highly charged analyte molecules.
Atmospheric pressure chemical ionization (APCI) is a relative of ESI. The ion source is similar to the ESI ion source. In addition to the electro hydrodynamic spraying process, a plasma is created by a corona-discharge needle at the end of the metal capillary. In this plasma, proton transfer reactions and possibly a small amount fragmentation can occur. Depending on the solvents, only quasi-molecular ions like [M+H]+, [M+Na]+and M+(in the case of aromatics), and/or fragments can be produced. Multiply charged molecules, as in ESI, are not observed.
ESI and APCI ionization sources are used almost exclusively for introduction of samples in a liquid flow.
Atmospheric pressure photoionization (APPI) is a complement to ESI and APCI by expanding the range and classes of compounds that can be analyzed, including nonpolar molecules that are not easily ionized by ESI or APCI. The mechanism of photoionization —ejection of an electron following photon absorption by a molecule—is independent of the surrounding molecules, thereby reducing ion suppression effects.
In addition, Plasma and glow discharge, thermal ionization and spark ionization are also used in mass spectrometry.
A few emerging techniques may allow ions to be generated under ambient conditions and then collected and analyzed by mass spectrometry. These techniques do not require sample pretreatment and can be performed under ambient conditions from any surfaces. These techniques include desorption electrospray ionization (DESI), the direct analysis in real time (DART), electrospray-assisted laser desorption/ionization (ELDI) and atmospheric solids analysis probe (ASAP).
The desorption electrospray ionization (DESI) technique involves directing a pneumatically-assisted electrospray, i.e. a fine spray of charged droplets, onto a surface bearing an analyte and collecting the secondary ions generated by interaction of the charged micro-droplets from the electrospray with the neutral molecules of the analyte present on the surface (See e.g., R. Graham Cooks, Zheng Quyang, Zoltan Takats, Justin M. Wiseman, Science, 311,1566, 2006). The ions are then delivered into mass spectrometer and are analyzed.
The direct analysis in real time (DART) technique is based on the reactions of electronic or vibronic excited-stat species, i.e. reagent molecules and polar or nonpolar analytes present in the ambient conditions (See e.g. Robert B. Cody, James A. Laramee and H. Dupont Durst, Anal. Chem. 77, 2297, 2005). In the DART method, an electrical potential is applied to a gas, typically nitrogen or helium, to form a plasma of excited-stat atoms and ions. After the ions are removed, the gas flow with the electronic or vibronic excited-stat species is directed toward a liquid or solid sample on a surface. Through the reaction, the sample ions are generated and moved into a mass spectrometer to be analyzed. (See U.S. Pat. No. 6,949,741).
Electrospray-assisted laser desorption/ionization (ELDI) uses a laser for desorption of neutral molecules on a surface and use a post-ionization of electrospray (See e.g. Jentaie Shiea, Min-Zon Huang, Hsiu-Jung Hsu, Chi-Yang Lee, Cheng-Hui Yuan, Iwona Beeth and Jan Sunner, Rapid Commun. Mass Spectrom. 19, 3701, 2005). Analytes are desorbed from solid metallic and insulating materials under ambient condition. Post-ionization of electrospray produces sample ions to be analyzed by a mass spectrometer.
Atmospheric solids analysis probe (ASAP) uses a heated gas jet directing onto a sample surface (See e.g. Charles N. McEwen, Richard G. McKay, and Barbara S. Larsen, Anal. Chem. 77, 7826, 2005). The desorbed species are ionized by corona discharge in the heated gas stream.
Mass analysis in a mass spectrometer can be performed using various mass analyzers that are based on different combinations of electric and/or magnetic fields. A magnetic sector analyzer analyzes ion mass using a static magnetic field to disperse ions according to ion mass. A quadrupole mass filter or quadrupole ion trap (QIT) or quadrupole linear ion trap (LIT) analyzer uses the stability or instability of ion trajectories in a dynamical electric RF field to separate ions according to their different m/z ratios. The quadrupole filter consists of four parallel metal rods. Both radio frequency (RF) voltages and direct current (DC) voltages with opposite polarities are applied across two pair of rods. Ions travel down the quadrupole in between the rods. Only ions of a certain m/z will reach the detector for a given ratio of RF and DC voltages: other ions have unstable oscillations and will collide with the rods. A quadrupole ion trap (QIT) mass analyzer is composed of a metal ring electrode and a pair of opposite metal end cap electrodes. The inner surfaces of the ring and two end cap electrodes are rotationally symmetric hyperboloids. Mass ion is trapped and then analyzed by so-called mass scanning methods.
In a linear ion trap, ions are confined radially by a two-dimensional (2D) RF field and axially by static DC potentials. In contrast to a three-dimensional (3D) ion trap, ions are not confined axially by RF potentials in a linear ion trap. A linear ion trap has a high acceptance since there is no RF quadrupole field along the z-axis. Ions admitted into a pressurized linear quadrupole undergo a series of momentum dissipating collisions effectively reducing axial energy prior to encountering the end of electrodes, thereby enhancing trapping efficiency. A larger volume of the pressurized linear ion trap relative to the 3D device also means that more ions can be trapped. Radial containment of ions within a linear ion trap focuses ions to a line, while the 3D ion trap tends to focus the trapped ions to a point. It has been recognized that ions can be trapped in a linear ion trap and mass selectively ejected in a direction perpendicular to the central axis of the trap via radial excitation techniques, or mass selective axially ejected in the presence of an auxiliary quadrupole field.
A Fourier Transformation Ion Cyclotron Resonance (FT-ICR) mass analyzer is based on the principle of ion cyclotron resonance. An ion placed in a magnetic field will move in a circular orbit at a frequency characteristic of its m/z value. Ions are excited to a coherent orbit using a pulse of radio frequency energy, and their image charge is detected on receiver plates as a time domain signal. Fourier transformation of the time domain signal results in the frequency domain FT-ICR signal which, on the basis of the inverse proportionality between frequency and m/z, can be converted to a mass spectrum.
A Time-of-flight (TOF) mass analyzer separates ions by m/z in a field-free region after accelerating ions to a constant kinetic energy. This acceleration results in any given ion having the same kinetic energy as any other ion. The velocity of the ion will however depend on the mass. The time that it subsequently takes for the particle to reach a detector at a known distance is measured. This time will depend on the mass of the particle (heavier particles reach lower speeds). From this time and the known experimental parameters one can find the mass of the particle.
Tandem mass spectrometry, which is widely applied, involves at least two steps of mass selection or analysis, usually with some form of fragmentation in between. Coupling two stages of mass analysis (MS/MS) can be very useful in identifying compounds in complex mixtures and in determining structures of unknown substances. In product ion scanning, the most frequently used MS/MS mode, product ion spectra of ions of any chosen m/z value represented in the conventional mass spectrum are generated. From a mixture of ions in the source region or collected in an ion trap, ions of a particular m/z value are selected in the first stage of mass analysis. These “parent” or “precursor” ions are fragmented and then the product ions resulting from the fragmentation are analyzed in a second stage of mass analysis. If the sample is a mixture and soft ionization is used to produce, for example, predominantly [M+H]+ions, then the second stage of MS can be used to obtain an identifying mass spectrum for each component in the mixture. For sector, quadrupole and time-of-flight instruments, each stage of mass analysis requires a separate mass analyzer.
A triple quadrupole mass spectrometer uses three quadrupole/multipole devices. The first quadrupole mass analyzer is used for parent ion selection, the second multipole collision cell is used for fragmentation and the third quadrupole is used for analyzing the fragmentation (daughter) ions. The quadrupole/TOF hybrid mass spectrometer, or Q-TOF, replaces the third quadrupole in triple quadrupole with TOF analyzer to give higher resolution and better mass accuracy. For quadrupole ion trap or ICR mass spectrometers, the MS/MS experiment can be conducted sequentially in time within a single mass analyzer. Ions can be selectively isolated, excited and fragmented, and analyzed sequentially in the same device. In addition, hybrid mass spectrometers may include a quadrupole linear ion trap combined with quadrupole ion trap (q-QIT), a quadrupole linear ion trap with FT-ICR, or an quadrupole ion trap with time-of-flight (QIT-TOF).
Consideration is now given improving the design of mass spectrometers. In particular, attention is directed to apparatus and methods of ion formation of surface adsorbed chemical species for mass spectrometry.